Re: [PATCH v7 31/41] x86/shstk: Introduce routines modifying shstk

[Edgecombe, Rick P]

On Thu, 2023-03-09 at 17:48 +0100, Borislav Petkov wrote:
> On Mon, Feb 27, 2023 at 02:29:47PM -0800, Rick Edgecombe wrote:
> > From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > 
> > Shadow stacks are normally written to via CALL/RET or specific CET
> 
>                                        ^
>                                        indirectly.

Dunno here, RSTORSSP/SAVEPREVSSP are kind of direct.

> 
> > instructions like RSTORSSP/SAVEPREVSSP. However during some Linux
> > operations the kernel will need to write to directly using the
> > ring-0 only
> 
> "However, sometimes the kernel will need to..."

Ok.

> 
> > WRUSS instruction.
> > 
> > A shadow stack restore token marks a restore point of the shadow
> > stack, and
> > the address in a token must point directly above the token, which
> > is within
> > the same shadow stack. This is distinctively different from other
> > pointers
> > on the shadow stack, since those pointers point to executable code
> > area.
> > 
> > Introduce token setup and verify routines. Also introduce WRUSS,
> > which is
> > a kernel-mode instruction but writes directly to user shadow stack.
> > 
> > In future patches that enable shadow stack to work with signals,
> > the kernel
> > will need something to denote the point in the stack where
> > sigreturn may be
> > called. This will prevent attackers calling sigreturn at arbitrary
> > places
> > in the stack, in order to help prevent SROP attacks.
> > 
> > To do this, something that can only be written by the kernel needs
> > to be
> > placed on the shadow stack. This can be accomplished by setting bit
> > 63 in
> > the frame written to the shadow stack. Userspace return addresses
> > can't
> > have this bit set as it is in the kernel range. It is also can't be
> > a
> 
> s/is //

Yep, thanks.

> 
> > valid restore token.
> 
> ...
> 
> > diff --git a/arch/x86/include/asm/special_insns.h
> > b/arch/x86/include/asm/special_insns.h
> > index de48d1389936..d6cd9344f6c7 100644
> > --- a/arch/x86/include/asm/special_insns.h
> > +++ b/arch/x86/include/asm/special_insns.h
> > @@ -202,6 +202,19 @@ static inline void clwb(volatile void *__p)
> >                : [pax] "a" (p));
> >   }
> >   
> > +#ifdef CONFIG_X86_USER_SHADOW_STACK
> > +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
> > +{
> > +     asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
> > +                       _ASM_EXTABLE(1b, %l[fail])
> > +                       :: [addr] "r" (addr), [val] "r" (val)
> > +                       :: fail);
> > +     return 0;
> > +fail:
> > +     return -EFAULT;
> 
> Nice!
> 
> > +}
> > +#endif /* CONFIG_X86_USER_SHADOW_STACK */
> > +
> >   #define nop() asm volatile ("nop")
> >   
> >   static inline void serialize(void)
> 
> ...
> 
> > +static int put_shstk_data(u64 __user *addr, u64 data)
> > +{
> > +     if (WARN_ON_ONCE(data & BIT(63)))
> 
> Dunno, maybe something like:
> 
> /*
>  * A comment explaining what that is...
>  */
> #define SHSTK_SIGRETURN_TOKEN   BIT_ULL(63)
> 
> or so?
> 
> And use that instead of that magical bit 63.

Seems very reasonable. Since we are calling this the "data format", I
might go with SHSTK_DATA_BIT.